zebra: use SO_RCVBUFFORCE for netlink socket

so net.core.rmem_max must not be adjusted. Requires
linux kernel >= 2.6.14, falls back to SO_RCVBUF on error

Signed-off-by: Ulrich Weber <ulrich.weber@sophos.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>

doc/zebra.8

@@ -80,7 +80,7 @@ handle flood of netlink messages from kernel. If you ever see "recvmsg overrun"
 messages in zebra log, you are in trouble.
 
 Solution is to increase receive buffer of netlink socket. Note that kernel
-doesn't allow to increase it over maximum value defined in
+< 2.6.14 doesn't allow to increase it over maximum value defined in
 \fI/proc/sys/net/core/rmem_max\fR. If you want to do it, you have to increase
 maximum before starting zebra.
 

zebra/rt_netlink.c

@@ -101,6 +101,10 @@ set_ifindex(struct interface *ifp, unsigned int ifi_index)
   ifp->ifindex = ifi_index;
 }
 
+#ifndef SO_RCVBUFFORCE
+#define SO_RCVBUFFORCE  (33)
+#endif
+
 static int
 netlink_recvbuf (struct nlsock *nl, uint32_t newsize)
 {
@@ -117,8 +121,16 @@ netlink_recvbuf (struct nlsock *nl, uint32_t newsize)
       return -1;
     }
 
-  ret = setsockopt(nl->sock, SOL_SOCKET, SO_RCVBUF, &nl_rcvbufsize,
+  /* Try force option (linux >= 2.6.14) and fall back to normal set */
+  if ( zserv_privs.change (ZPRIVS_RAISE) )
+    zlog_err ("routing_socket: Can't raise privileges");
+  ret = setsockopt(nl->sock, SOL_SOCKET, SO_RCVBUFFORCE, &nl_rcvbufsize,
 		   sizeof(nl_rcvbufsize));
+  if ( zserv_privs.change (ZPRIVS_LOWER) )
+    zlog_err ("routing_socket: Can't lower privileges");
+  if (ret < 0)
+     ret = setsockopt(nl->sock, SOL_SOCKET, SO_RCVBUF, &nl_rcvbufsize,
+		      sizeof(nl_rcvbufsize));
   if (ret < 0)
     {
       zlog (NULL, LOG_ERR, "Can't set %s receive buffer size: %s", nl->name,